Among conventional semiconductor light emitting elements capable of tuning a laser oscillation wavelength, there is, for example, a semiconductor light emitting element in which a laser cavity is formed of a reflection edge and an optical reflector including, for example, a chirped diffraction grating and electrodes.
Moreover, among semiconductor lasers, there is, for example, a semiconductor laser which includes a front DFB region and a back DFB region or a DBR region and which includes low-reflectivity film on both edges.
Furthermore, among semiconductor laser devices, there is, for example, a semiconductor laser device which includes a wavelength monitoring region and a semiconductor laser region having a gain region, a DBR region, and an amplifying region and which includes an anti-reflection film on an output edge of the semiconductor laser region.
For example, among optical modules used in an optical transmission system, there is an optical module including a wavelength tunable laser, a multi-wavelength light source, and the like.
In order for such an optical module to increase a side mode suppression ratio of an oscillation mode and achieve the stability of the oscillation mode, the optical element may be equipped with a distributed Bragg reflector (DBR mirror) in a band pass filter form instead of an edge reflector mirror. The DBR mirror has reflectivity which is flat and definite in a used region and which decreases sharply outside the used region.
The DBR mirror in the band pass filter form described above can achieve a filter form to some extent by using a diffraction grating which has a fixed coupling coefficient and a first-order periodic structure where the period is chirped (gradually varied).
However, even though the period of the diffraction grating is simply chirped, a region with a reflectivity of about 30% has several percent of ripples, so that a gain difference between a selected mode and a side mode may not be obtained in mode selection.
As a method of removing the unnecessary ripples, it is conceivable to use a diffraction grating in which: the DBR region is divided into segments; the diffraction grating period is varied substantially linearly across these segments; and the coupling coefficient gradually decreases from the maximum value to zero in directions toward both ends of the DBR region.
A substantially flat reflection characteristic in which ripples are theoretically suppressed is thereby obtained.
In this case, a conceivable method for reducing the coupling coefficient to zero is varying the duty ratio of a diffraction grating of a first-order period from 50% to 100% or from 50% to 0%.
However, when the duty ratio of the diffraction grating of the first-order period is varied from 50% to 100% or from 50% to 0% to reduce the coupling coefficient to zero, the width in the diffraction grating (width of the ridge of the diffraction grating) is gradually reduced or increased.
Accordingly, as the coupling coefficient becomes closer to zero, the diffraction grating becomes more difficult to form by etching, or more likely to be deformed or disappear.
This causes variation in equivalent index, and the reflection characteristic is deviated from the designed characteristic.
The following are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 2009-59729,    [Document 2] Japanese Laid-open Patent Publication No.2004-241627,    [Document 3] Japanese Laid-open Patent Publication No.2011-49317 and    [Document 4] Dug-Bong Kim et al., “Fabrication of Sidelobe-Suppressed InP—InGaAsP Vertical Coupler Optical Filter Using Pair Grating Structure”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 11, pp. 1593-1595, NOV. 1998.